Question:If a pure man marries a woman who he thinks is a pure woman (virgin), and he finds out later she wasn't a virgin when they married, is he condemned to hell because it wasn't a pure and holy marrage, even though he is a member of the true New Testament church?
God placed no requirement on marriage that only virgins may marry. As an example, the prophet Hosea was specifically told to marry a prostitute to illustrate several points. "When the LORD began to speak by Hosea, the LORD said to Hosea: "Go, take yourself a wife of harlotry and children of harlotry, for the land has committed great harlotry by departing from the LORD"" (Hosea 1:2). His marital problems that resulted served as illustrations for God's problems with the Israelites. However, his marriage was still a legitimate marriage.
Also, if only virgins could marry, then a widow could not remarry. However, "Therefore I desire that the younger widows marry, bear children, manage the house, give no opportunity to the adversary to speak reproachfully" (I Timothy 5:14).
The purity of marriage is found in how well a couple honors their marriage covenant, not in their behavior prior to marriage.
For that matter, purity in the Bible isn't just about sexual sins. It is about being free from sins. "Do not lay hands on anyone hastily, nor share in other people's sins; keep yourself pure" (I Timothy 5:22). From that viewpoint, no one has ever entered marriage perfectly pure because we all sin (Romans 3:23). The point is not whether we have sinned, but whether we have rejected sin and sought forgiveness from God. He then cleanses us from unrighteousness. "If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness" (I John 1:9).
The sexual past of your chosen spouse has no effect on your salvation. If your spouse commits adultery in the future, it has no impact on whether you go to heaven or not. "The soul who sins shall die. The son shall not bear the guilt of the father, nor the father bear the guilt of the son. The righteousness of the righteous shall be upon himself, and the wickedness of the wicked shall be upon himself" (Ezekiel 18:20). Just as you cannot get to heaven because of your spouse, your spouse cannot send you to hell either.
What will impact your salvation, however, is whether you can forgive your wife for the imaged wrong you see against her. "But if you do not forgive men their trespasses, neither will your Father forgive your trespasses" (Matthew 6:15).